The present invention relates to logic circuits that store digital information on a temporary basis and, more particularly, to shift registers that store and shift a number of binary or decimal digits.
Shift registers for storing and shifting digital data or binary digits as well known in the art and are used in myriad applications. A basic shift register can be comprised of any number of cascaded flip-flops (FF's) which may shift their contents one position to the right upon each occurrence of a clock pulse, i.e., each flip-flop stage switches states once for each clock pulse. By using added gating, the basic shift register can be made to shift digital data to the right until some occurrence happens upon which the data is then shifted to the left. This type of shift register is known as a shift-right shift-left register. Potential applications for such shift registers, among others, are as counters.
For example, a shift-right shift-left register may be utilized in an electronic telephone integrated circuit to generate telephone dial tones from digital generated signals associated with dual-tone multi-frequency (DTMF) dialers as is understood. Typically, shift registers compatible with I2L technology have been used in such electronic telephoneintegrated circuits. These I2L shift registers have been implemented using D flip-flops or J-K flip-flops. D flip-flops and J-K flip-flops require a certain number of gates to function; which for a particular application may be excessive. For instance, a typical I2L D flip-flop requires the use of seven inverter gates (see U.S. Pat. No. 4,085,341). Thus, for an eight stage shift register (eight D flip-flops are required) using a conventional I2L D flip-flop requires 56 gates.
In many integrated circuit applications the function of the circuit is becoming more and more complex which means that more and more circuit components are required. If the number of components required for a particular function can be reduced the die area needed for the integrated circuit is reduced. By reducing die area means, (1) that additional circuit functions can be integrated into the integrated circuit, and (2) fabrication costs are reduced. Hence, if the number of gates required to perform the function of the shift registers can be reduced the aforementioned desired goals can be achieved.
Thus, a need exists for an improved shift register which utilizes fewer component count than is available in the prior art.